It is known in technical literature that HDBR lasers are generally formed by an active element, comprising a semiconductor chip in which a Fabry-Perot semiconductor laser diode, or a Semiconductor Optical Amplifier (SOA), is integrated, and a reflector. A facet of the laser diode or, respectively, of the SOA, facing a tip or termination of an optical fibre to which the active element is coupled, is coated with a layer of anti-reflection material. The other facet of the laser diode chip, respectively the SOA, opposite to that facing the fibre termination, is conventionally coated with a layer of reflective material. The reflector comprises a Bragg grating, formed in said optical fibre.
Such lasers generally have a rather pure Continuous Wave (“CW”) emission spectrum, and are mainly used as laser pumps for optical amplifiers. A review of the possible applications of this type of laser is provided in C. R. Giles, “Lightwave Applications of Fiber Bragg Grating”, Journal of Lightwave Technology Vol. 15, No. 8, August 1997, pages 1391 and following.
The main features of this type of laser depends on the overall cavity length, given by the distance between the facet of the active element coated with the reflecting material and the position, along the fibre, of the equivalent mirror of the Bragg grating. The overall cavity length is thus given by the sum of the active element length, the distance between the active element facet coated with the anti-reflecting material and the fibre termination faced thereto, and the distance between the fibre termination and the position, along the fibre, of the equivalent mirror of the Bragg grating. Such a position is located at the point, in the grating, from where the photons that are reflected towards the active element have a time of flight equal to that of the photons sent by the active element towards the grating; in other words, the position of the equivalent mirror is the position wherein a mirror would have to be positioned in order that a pulse sent by a light source and reflected by the mirror returns to the source in the same time the pulse sent into the grating would take to return.
Since in general the width of the modulation band (modulation bandwidth) of a laser depends on the time of flight of the photons within the cavity of the laser, and since the time of flight of the photons within the cavity increases with the cavity length, a relation exists between the cavity length and the laser modulation bandwidth: the lower the cavity length, the higher the laser modulation bandwidth.
As regards HDBR lasers, for the very reason that the cavity is external to the active element, so that the cavity length is rather high compared to other types of laser, an existing technical prejudice wants that these lasers can advantageously be used in CW applications, but not in applications wherein the possibility of directly modulating the laser is to be contemplated; this because the laser modulation bandwidth, for the above considerations on the cavity length, would be always narrow and it would not be possible to reach modulation frequencies of interest for the current applications.
In accordance with such a technical prejudice, EP-A-0 949 729 confirms that the main parameter for realizing an HDBR laser for direct modulation is the length of the laser cavity. In that patent application, with the aim of demonstrating the feasibility of operating external cavity lasers not only in CW, but also in direct modulation, a solution is proposed that allows for reducing the cavity length. To such purpose, it is suggested to realize, in the optical fibre coupled with the active element, gratings having a half-gaussian or anyhow asymmetric spatial profile of modulation of the refractive index longitudinally to the fibre. In this way, according to the teachings provided, the equivalent mirror of the grating can be positioned sufficiently close to the fibre termination which is optically coupled to the active element.
In the case described in such patent application, the position of the equivalent mirror of the grating can actually be made close, approximately 2 mm, to the fibre termination coupled to the laser diode. On the other hand, in the same document it is shown that the distance of the equivalent mirror from the fibre termination is heavily affected by the length of the fibre portion, approximately 3 mm, concerned by the attachment of the fibre termination in front of the laser: the Bragg grating can in fact-be formed only downstream of said fibre portion, the length of which cannot be reduced below a given limit due to the fact that, for the attachment, resins are used. The overall cavity length thus results to be approximately 5 mm.
With such a laser cavity length value, in the above mentioned patent application it is stated that the upper limit to the laser modulation bandwidth is approximately equal to 14 GHz.
However, experiments conducted by the Applicant of the present patent application evidenced that such an upper limit is merely theoretical being related to small signal modulation, since it does not take into account factors which are instead essential for having a good quality of transmission, such as the laser mode stability when the laser is directly modulated with NRZ modulation format.
Laser mode stability is essential in applications in the field of digital communication systems, in which it is of paramount importance to have a very low transmission bit error rate. In such application field it is in fact absolutely necessary to guarantee a Bit Error Rate (“BER”) not higher than 10−9 or 10−10, and in the design phase it is aimed at guaranteeing an even lower BER, ranging from 10−12 and 10−14.
The same experiments previously mentioned have instead proven that, using a spatially asymmetric, e.g. half-gaussian, grating as proposed in the above mentioned patent application, the maximum frequency of direct NRZ modulation format of the laser which allows to comply with the low BER requirement is not higher than approximately 1 Gbit/sec. Therefore, the laser cavity structure proposed in such European patent application allows to obtain a laser with high spectral purity, that can also be directly modulated in real conditions, but with the constraint that the maximum bit rate of direct modulation does not exceed 1 Gbit/sec.
Such a bit rate, considerable in absolute sense, does not however coincide with any of the values prescribed by the standard SDH (“Synchronous Digital Hierarchy”) or the U.S. counterpart SONET. For example, the SDH standard prescribes bit rate values which are multiple of four times of a base frequency of approximately 155 MHz (thus, 622 MHz, 2.5 GHz, 10 GHz and so on).
It is of paramount interest to have a laser with high spectral purity, that can be directly modulated at high bit rate still remaining stable in wavelength, that is, without errors in the transmission. In particular, in view of the requirements prescribed by the SDH or SONET standards, it is of the outmost interest to have a laser that can be directly modulated at a bit rate of at least 2.5 Gbit/sec, or multiple of four times thereof.